Process for distributing the stock flowing through a paper machine head box

摘要

In a process for distributing a suspension of fibrous material (stock) in the head box of a paper machine, different additive materials are injected into the stock at two or more different levels relative to the plane of the paper. Also claimed is a process in which different stock streams are supplied through two or more separate distributors, guiding devices, nozzles and slots in the head box. The stock supplied to form the outer (i.e. surface) layers of the paper contains: (i) fibres with a higher degree of milling; (ii) more filler or fines; or (iii) fibrous material which has been refined with a lesser amount of chemicals and has stronger fibres than the stock forming the inner layers. Further claimed is a head box in which the processes are carried out. Preferably the additives may be injected into the stock in the middle section (12), the nozzle (14), and/or the distributor (11) parts of the head box. The additives are fillers, chemicals (especially retention media), starch, suspensions of fibrous materials, or mixtures thereof. Less retention medium and/or less filler and/or fines are added to the centre of the stock flow than to the outer layers. The additives supplied to the centre of the web are those which increase strength, and those supplied to the outer layers increase printability. The concentration of the fibrous material in the stock may be adjusted in zones across the web width, whilst the volume flow rate of the suspension is kept substantially constant. Similarly, the additive streams may be adjusted in zones across the machine width and/or in the direction perpendicular to the web surface volume whilst flow rate is kept constant by the addition, as necessary, of water.